Generic assertion macro. 'form' is any predicate test. 'msg' is an optional message to attach to the assertion. Example: (is (= 4 (+ 2 2)) "Two plus two should be 4") Special forms: (is (thrown? c body)) checks that an instance of c is thrown from body, fails if not; then returns the thing thrown. (is (thrown-with-msg? c re body)) checks that an instance of c is thrown AND that the message on the exception matches (with re-find) the regular expression re.